As is known, now available are numerous metrology systems, which find use, amongst other things, in the aerospace sector.
In particular, metrology systems are known having the function of determining the attitude and/or position of a satellite. Even more in particular, metrology systems are known which enable, given a pair of satellites, determination of the mutual position and mutual attitude of the two satellites.
Determination of the attitudes and positions of satellites is of particular importance in the case of satellite systems the satellites of which are arranged in formation, i.e., in the cases where there is envisaged the determination of the attitude and position of each satellite as a function of the attitude and position of the other satellites.
In practice, given two satellites, determination of the mutual attitude and mutual position requires determination of six degrees of freedom. In fact, assuming a first reference system and a second reference system fixed with respect to a first satellite and a second satellite, respectively, and formed, each, by a respective triad of perpendicular axes, the mutual attitude and mutual position of the first and second reference systems, and hence of the first and second satellites, can be expressed in terms of three (linear) displacements and three rotations (angles). In particular, the mutual position of the first satellite with respect to the second satellite can be expressed by means of a set of three displacements measured, respectively, along the three axes of the second reference system. Likewise, the attitude of the first satellite with respect to the second satellite can be expressed by means of a set of three angles, equal to corresponding rotations of the first reference system with respect to the second reference system.
This being said, in general optical metrology systems now available can be divided into so-called “coarse” systems and so-called “fine” systems, according to the accuracy and the field of application, the latter being given by the range of distances that can lie between the satellites without the levels of performance degrading significantly.
In greater detail, fine metrology systems enable determination of the mutual position of two satellites with an accuracy lower than a centimeter, provided that the satellites are not set at a distance apart greater than about fifty meters. Some fine metrology systems even enable determination of the mutual position of two satellites with an accuracy of the order of one tenth of a millimeter, provided that the satellites are not set at a distance apart greater than one meter.
Instead, coarse metrology systems are characterized by an accuracy not lower than about ten centimeters. However, they are able to operate also when the distance between the satellites is greater than fifty meters, for example also up to distances of twenty kilometers.
By way of example, coarse metrology systems comprise metrology systems based upon the use of the satellite global positioning system (GPS), as well as metrology systems based upon the use of radio-frequency radiation, the latter resorting to considerably complex antenna networks.
As regards, instead, fine metrology systems, known to the art are systems of an at least in part projective type, which envisage that, given two satellites, one of them is equipped with a target formed by a number N of light sources, and the other is equipped with an optical unit, which includes an optoelectronic sensor able to acquire an image of the target, on the basis of which, by means of post-processing, the optical unit itself determines one or more of the aforementioned degrees of freedom.
By way of example, the patent application No. EP1986018 describes a system for determining the position and attitude of a system with six degrees of freedom, and where the number N of light sources of the target is equal to one. However, to enable determination of all six degrees of freedom, the system described in the document No. EP1986018 requires the individual light source to be formed by a coherent-light source such as, for example, a laser, and moreover requires that the optical unit will be able to carry out, in addition to processing the images of the target, measurements of the power effectively received by the optoelectronic sensor and an angular measurement of rotation of the polarization of the beam of light emitted by the coherent-light source.
In even greater detail, with reference to a first satellite and a second satellite, and assuming that the target is located on the first satellite, the system described in the document No. EP1986018 envisages that the optical unit on board the second satellite will be equipped with three optoelectronic detectors that are able to detect, each, the power associated to the fraction of electromagnetic radiation emitted by the coherent-light source and effectively impinging upon the optoelectronic detector itself. Consequently, the system described in the document No. EP1986018 is not of a purely projective type.
There are on the other hand known fine metrology systems that do not envisage determination of measurements of power, i.e., metrology systems of a purely projective type. An example of said metrology systems is provided in the document No. U.S. Pat. No. 7,561,262, where the light sources are formed by reflectors designed to be arranged on the first satellite, which are illuminated by radiation emitted by the second satellite. Moreover known are fine metrology systems of a purely projective type, where the target is formed by a particularly large number of light sources (for example, eight).
In addition, stereoscopic-vision systems are known, such as for example the system described in the document No. FR2702056, where two different optical paths are provided, which are characterized by one and the same focal length and are optically connected to one and the same sensitive surface. In this way, for each object, formed on the sensitive surface are two corresponding images, which define a three-dimensional image of the object.
Moreover available are systems designed to determine the shape of the surface of a target. For example, the system described in the document No. US2011/0043808 comprises a device designed to irradiate the target, as well as a display device designed to receive the radiation reflected by the target and to convey it towards a plurality of sensitive surfaces.
Even though fine metrology systems are characterized by a high degree of accuracy, unlike coarse metrology systems, they can operate in an optimal way only if the distances between the satellites are limited, as mentioned previously. Consequently, there is felt the need of having available a metrology system capable of operating both in the case where the distance between the target and the optical unit is limited and in the case where the distance between the target and the optical unit is large. In this connection, the measuring and tracking system described in the document No. US2008/0111985 comprises two optical paths, which share one and the same optical input, but terminate on two different photosensitive surfaces and have different focal lengths. In this way, the measuring and tracking system extends the range of the distances between the target and the optical unit within which the system itself is characterized by a high accuracy.
In practice, since each image is formed by a plurality of corresponding image dots, the measuring and tracking system described in the document No. US2008/0111985 resorts to two different photosensitive surfaces in order to acquire the two different images correctly, i.e., in order to associate each image dot to the respective image, thus enabling a correct processing of the images themselves. In this way, it is not necessary to use processing algorithms or additional hardware designed to determine which of the image dots will belong to a first image and which to a second image. However, the use of two photosensitive surfaces becomes in effect necessary.